Parent Stars Can Smother Planets

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For a planet to be habitable, one parent star seems to be a
better bet than two, especially if the stars are physically very
close.

So concludes researchers who discovered what they believe to be
the rocky remains of planets around three pairs of closely
orbiting binary stars.

"It really only takes one rogue planet to have its orbit
perturbed and that can wreak havoc on the whole system," Jeremy
Drake, with the with Harvard-Smithsonian Center for Astrophysics
in Cambridge, Mass., told Discovery News.

Drake and colleagues found warm dust disks around sun-like stars
that should have long ago dissipated any clouds of debris left
over from their formative years.

"The fact that they're old, mature systems must mean that they've
made the stuff relatively recently," said Drake.

Planets have been found in systems with two stars, but those
stars' orbits are more stable. The type of binary stars targeted
in the new study are located only about 2 million miles from
their partners -- about 2 percent of the distance between the sun
and Earth.

At that range, powerful magnetic fields and strong stellar winds
slow the stars' spin, causing them to move even closer together
over time. The resulting gravitational disturbances would impact
any planets in tow and eventually cause the orbiting bodies to
crash into one another.

Astronomers, who used NASA's Spitzer Space Telescope for their
study, are hoping to get a more detailed look at the debris rings
with another infrared observatory named Herschel. They also plan
to develop computer models to better understand the destabilizing
effects of the stars on any planets.

The discovery of these late-stage debris rings doesn't bode well
for the existence of habitable planets in these systems, adds
Drake.

"You could be lucky. You could be not. It is possible that a
planetary system in a well evolved state could, over time, be
driven to a destructive phase," he said.

NASA's Marc Kuchner, with the Goddard Space Flight Center in
Maryland, thinks planets orbiting farther from the parent stars
would have a better shot at survival.

"The region near the stars is very unstable, but if you were to
go far enough out I'd expect there'd be a large region of
stability," Kuchner told Discovery News.

"On these planets, there would be two suns in the sky. At least
one sun would be very luminous, highly variable and magnetically
active, which might make the surface of the planet a hostile
place. But on a planet with oceans, there could be happy sea
creatures not too far beneath the protective water," he said.

The research appears in last week's issue of Astrophysical
Journal Letters.